The use of transistors as electronic switches is widespread. Owing to the high switching frequencies with which transistors can be operated, transistors are not only suitable as “static” switches that are closed over a relatively long period of time, such as a few seconds, minutes or hours, but are also suitable for the clocked or pulsed driving of loads.
Transistors driven in a clocked or pulsed manner are used for example in driver circuits for inductive loads, such as, for example, in half-bridge or full-bridge drivers for electric motors, solenoid valves, etc. A further field of use is switching converters or switch mode power supplies, in which a clocked driving of a transistor serves for regulating the current consumption and thus for regulating the output voltage. However, the present invention is not restricted to these applications.
For driving the transistor, usually by means of a logic unit a digital control signal is output and fed to a drive signal generator, which generates a drive voltage for the transistor depending on the digital control signal. The digital control signal specifies whether the transistor is intended to be switched on or to remain switched on, or whether it is intended to be switched off or to remain switched off. By way of example, a high level of the digital control signal may mean that the transistor is intended to be switched on, and a low level that the transistor is intended to be switched off.
If a transistor such as, for example, an IGBT or a MOSFET is switched on in normal operation, its load current I (i.e. the current through the load path between emitter and collector or between source and drain) is limited by the operating voltage VB, and the load amount (more precisely: the resistance RL thereof). The minimum voltage across the load path in the fully switched-on state of the transistor is also referred to as the forward or saturation voltage VCE,SAT. For the power loss PV dissipated in the transistor in normal operation it holds true that: PV=I·VCE,SAT. In the switched-on state, for a given gate voltage VGE (i.e. in an IGBT the voltage between gate and emitter or in a MOSFET the voltage between gate and source), the transistor can carry a specific maximum load current I, which is also referred to as saturation current ISAT. The saturation current ISAT is thus dependent on the gate voltage VGE chosen.
Since, in the case of a short circuit, the full operating voltage VB is dropped across the load path of the transistor (VCE=VB), the load current I normally rises up to the saturation current ISAT. For the power loss PMAX during short-circuit operation, i.e. if a short circuit of a load connected in series with the load path is present, it holds true that: PMAX=VB·ISAT. For the energy EMAX dissipated in short-circuit operation it holds true that: EMAX=VB·ISAT·tSC, wherein tSC denotes the time between the occurrence of the short circuit of the load and the switching off of the transistor. In order to prevent a thermal runaway of the transistor, the energy EMAX should remain below a critical energy ECRIT, since the transistor is otherwise destroyed on account of a very high hot leakage current as a result of thermal runaway.
The operating voltage VB is usually predefined by the application. The time tSC required for the detection of a short circuit and the subsequent switching off of the transistor is substantially dependent on external parameters. In principle, it is possible to design a transistor with a high short-circuit strength, such that tSC can be chosen to be relatively long. However, this is associated with a low saturation current ISAT and a high forward voltage, which results in higher losses in normal operation. Therefore, there is a conflict of aims between a good performance in normal operation, on the one hand, and the required short-circuit strength, on the other hand.
The saturation current ISAT and also the saturation voltage VCE,SAT of a transistor such as an IGBTs or MOSFETs are dependent, as already mentioned, on the gate voltage VGE present at the gate terminal of the transistor. If the gate voltage is only slightly greater than a switch-on threshold voltage Vth of the transistor, the saturation current ISAT is comparatively small and the saturation voltage VCE,SAT is comparatively large. The higher the gate voltage VGE applied to the gate terminal, the higher the saturation current ISAT becomes and the lower the saturation voltage VCE,SAT and the losses in normal operation are. The relationship between saturation voltage VCE, SAT and gate voltage VGE is nonlinear. Starting from a specific level of the gate voltage VGE a further increase no longer leads to a significant reduction of the saturation voltage VCE,SAT.
In order to detect a short circuit of a load it is known, in principle, to monitor by means of a short-circuit monitoring a measurement variable which allows the conclusion to be drawn about the occurrence of a short circuit of the load, and, if the monitoring of the measurement variable is indicative of the presence of a short circuit, to drive the transistor in the off state and thereby to switch it off (protective switch-off). However, in specific situations it happens that the short-circuit monitoring incorrectly signals a short circuit of the load, without a short circuit of the load actually being present. In these cases, the transistor would be switched off unnecessarily.
By way of example, the short-circuit monitoring can use the current through the load path of the transistor as a measurement variable. If capacitances electrically connected to the transistor, e.g. capacitors or parasitic capacitances such as electrical terminal and connection lines, are then charged or subjected to charge reversal on account of the transistor being switched on, it may be that the current through the load path of the transistor temporarily rises to values that are higher than the maximum permissible continuous current through the transistor. Therefore, in order to avoid destruction of the transistor, the short-circuit monitoring outputs to the logic unit a signal which causes the transistor to be switched off in an unscheduled manner. In order to avoid such an unscheduled switching off (i.e. caused by a wrongly indicated short circuit of the load) of the transistor and to enable proper switching operation of the transistor, an unscheduled switching off of the transistor must be prevented for a certain waiting duration (typically: 10 μs) starting from the switching on of the transistor until, after the switching on, it can be unambiguously established whether a short circuit of the load is actually present. For the case where a short circuit of the load is already present when the transistor is switched on, and the short-circuit monitoring correctly decides on the presence of a short circuit of the load, the transistor must be able to withstand the waiting duration in the switched-on state (i.e. when its load path is conducting) without being damaged. In principle, transistors having a lower efficiency (i.e. transistors having higher on-state losses) could be used for this purpose, although this leads to undesired losses in normal operation. Therefore, the practice has been adopted of driving the transistor with a reduced drive voltage for a predefined waiting duration upon switch-on and increasing the drive voltage after the waiting duration has elapsed, provided that the short-circuit monitoring does not signal a short circuit once the waiting duration has elapsed. The predefined waiting duration and the reduced drive voltage are coordinated with one another such that the transistor is not damaged even if the load is short-circuited during the entire waiting duration. Since drive circuits of a specific type are usually operated in conjunction with a wide variety of hardware configurations, the capacitances are not fixed from the outset. Therefore, the waiting time must contain a sufficient safety margin. However, this has the effect that the transistor is driven with the reduced drive voltage during the waiting time even if the short-circuit monitoring does not signal a short circuit. This in turn results in operation with reduced efficiency.